Duplicate listings for houses on property websites

I've noticed while browsing the various property websites that the same house might be listed on more than one website - but with a different estate agent selling.

Can a house be on the books of more than one EA simultaneously?

Any idea what is going on here?

There is nothing to say that you can't advertise your house with as many estate agents as you want.

I thought most EA's looked for sole agency when selling a house.....

They do look for it, obviously its less complicated if an EA has sole agency and they are also guarnteed the full commission on sale of the property with no competition.

But there are disadvantages for the seller as well, two times the advertisings fees, and potentially additional EA fees also (say EA1 arranges viewings and a potential buyers makes an offer to EA2 on foot of the viewing arranged by EA1 - Does EA 1 have a valid claim for fees?).
The two EA's would need to be communicating well to ensure no crossover and if in competition with another EA would the EA devote more time to selling a different property the full fees for which they would get? I think so.